Though most prevalent today in developing third-world countries, parasitic infections plague industrialized countries as well. Parasitic infections may not be as common as infections of bacterial and viral origin, but they can cause tremendous human suffering.
Numerous types of parasitic infections are treated each year in the United States, with toxoplasmosis, giardiasis, trichomoniasis, and pinworm infections among the most frequently encountered. [See generally, Medical Microbiology--An Introduction to Infectious Diseases, John C. Sherris ed., Ch. 44 (1984)]. Immunocompromised patients, such as Acquired Immune Deficiency Syndrome (AIDS) sufferers, are especially susceptible to certain types of parasitic infections.
The term "parasitism" denotes a relationship wherein a host organism is harmed in some way by the activities of another organism, the parasite. Thus, parasitism is distinguishable from other relationships where both organisms benefit (mutualism) or where one organism benefits and the other organism is not harmed (commensalism). Parisitism entails a prolonged and intimate relationship between the involved organisms. [See generally, Edward K. Markell & Marietta Voge, Medical Parasitology, Ch. 2 (5th ed. 1981)].
The intestinal roundworms represent one classification of parasites. The intestinal roundworms are also referred to as the intestinal nematodes; hence, the two terms may be used interchangeably. The details of the intestinal roundworms' structural characteristics and reproductive properties are well known. [See, e.g., J. Walter Beck & John E. Davies, Medical Parasitology, Ch. 8 (2d ed. 1976); Medical Microbiology--An Introduction to Infectious Diseases, Ch. 49, supra; and Edward K. Markell & Marietta Voge, Medical Parasitology, Ch. 8, supra].
Six intestinal roundworms have traditionally been recognized as infecting humans, affecting about 25% of the world's population. These intestinal roundworms are Enterobius vermicularis (the pinworm), Trichuris trichuria (the whipworm), Ascaris lumbricoides (the large roundworm), Necator americanus and Ancyclostoma duodenale (the hookworms), and Strongyloides stercoralis. The adult forms of each can persist for years in the lumen of the human gut, and the severity of illness produced from these intestinal roundworms is dependent upon their extent of adaptation to the human host. The manifestations of intestinal roundworm infection include malnutrition, anemia, gastrointestinal disturbances, and even death. [See Medical Microbiology--An Introduction to Infectious Diseases, Ch. 49, supra].
The modalities used to treat roundworm infection primarily involve oral administration of a pharmaceutical agent. Several oral agents are available in the United States to treat roundworm infections, including mebendazole (Vermox.RTM., Janssen), piperazine citrate, pyrantel pamoate (Antiminth.RTM., Pfizer), and thiabendazole (Mintezol.RTM., Merck).
Most pharmaceutical agents are effective against more than one species of roundworm, though different treatment regimens are often employed against different organisms. To illustrate, mebendazole can be used to treat both pinworm infection and hookworm infection (as well as infections caused by other roundworms). The regimen for treating pinworm infection (enterobiasis) generally entails administration of a single 100 mg dose; some clinicians suggest administration of a second 100 mg dose two weeks later. By comparison, treatment of hookworm infection usually is performed by administration of 100 mg given twice daily for three consecutive days; this regimen is repeated if the patient is not cured within 3-4 weeks. Thus, though the same drugs are often effective against several types of roundworms, different treatment regimens are frequently required. [See generally, AHFS Drug Information 94, Gerald K. McEvoy, ed., pp.39-41 (1994)].
Numerous adverse effects have been reported following administration of pharmaceutical agents used to treat roundworm infections. Though the newer agents generally are associated with fewer and less severe adverse effects, the adverse effects experienced with these newer agents still can interfere with the subject's normal functioning. For example, diarrhea, abdominal pain, nausea, vomiting, dizziness, drowsiness, and weakness, among others, have all been reported with mebendazole, an agent generally recognized as causing minimal adverse effects.
Finally, the anthelmintic agents have not eliminated parasitic infections as a health concern, even in developed countries like the United States. Again, to use mebendazole as an example, egg reduction percentages and/or cure rates of over 90% are generally achieved in patients suffering from enterobiasis, ascariasis, and certain species of hookworm infection. Similar egg reduction rates, as well as cure rates of about 70%, have been produced by mebendazole treatment in whipworm infection (trichuriasis). Thus, even when a roundworm has been identified and the concomitant drug-of-choice has been administered, it is still uncertain whether all vestiges of the infection will be eliminated.
Though the treatment modalities currently being utilized to combat parasitic infections have had some success, they have not eradicated parasitic infections. Furthermore, individuals are sometimes intolerant to pharmaceutical agents that are generally effective against the organism they harbor. Individuals may also suffer from infections caused by parasites that are not susceptible to any presently available treatment modality. Clearly, new treatment methods and agents would be welcomed by those plagued by parasitic infections who either cannot tolerate available treatment regimens or who harbor resistant organisms.